Even natural brands' you might hope to be GF - such as Biona's - contain wheat flour, and popular products found in Holland & Barrett and other health stores - such as Panda Licorice and RJ's Natural Licorice - are also gluten-containing.
These, though, are safe:
Orgran Molasses Licorice. Uses soy flour instead of wheat flour, and is GF, milk free and egg free, but seems low in actual liquorice (0.5%). Available from Goodness Direct (UK), iHerb (US) and, internationally, via Amazon.
Amazon, and in the UK sold by Real Foods.
YumEarth Gluten Free Licorice. Fruit-flavoured (strawberry, pomegranate, peach) candy - so some might not count this as 'proper' liquorice! Can only find them in the US, sold on Amazon or on the YumEarth site directly.
Candy Tree GF Licorice. Don't know much about this US brand. Made with molasses, syrup and rice flour / starch - but can't see any actual liquorice in the ingredients (instead, 'natural flavour'). Browse online here.
In the UK, you might like to try some specialist online licorice stores:
All Things Liquorice has a GF section, including Lakrids (see above) and Italian brand Amarelli, renowned for the purity of its liquorice products and collectible tins.
Liquorice with a Twist also has a GF page, including a selection of Dutch products (apparently, the Dutch like their liquorice salty).
And another, Liquorice World, also has a GF collection. They too stock Lakrids and Amarelli.
In the US, there's this outlet:
Licorice International - which has a large GF section.